Carbodiimides can be produced from isocyanates in a particularly simple manner, even at room temperature, by the process which forms the basis of German Auslegeschrift No. 1,130,594 and U.S. Pat. Nos. 2,853,473 and 2,941,966. As disclosed therein, the most important and effective catalysts from the technical point of view, which very quickly carbodiimidize practically any aromatic mono- or polyisocyanate even at room temperature, and which convert unreactive aliphatic or cycloaliphatic mono- or poly-isocyanates into carbodiimides at temperatures above approximately 150.degree. C, are, for example, those corresponding to the formulae: ##STR2##
Such catalysts have already found technical application in the production of polycarbodiimide foams.
Unfortunately, hitherto it has not been possible to stop the carbodiimide formation when using the extremely soluble catalysts mentioned above. Consequently, it is difficult to obtain storage stable carbodiimides or poly-carbodiimides containing isocyanate groups. Moreover, it has not been possible with these catalysts to produce stable solutions of diisocyanato carbodiimides, such as ##STR3## in excess monomeric mono-, di-, or polyisocyanate.
The idea of deactivating the phospholine oxides which are highly effective as catalysts for the carbodiimidization reaction, by the addition of compounds which tend to form salts or adducts with the phospholine oxides, while promising in theory seemed fruitless, since it was known from German Offenlegungsschrift No. 2,245,634, that such adducts themselves constitute carbodiimidization catalysts for isocyanates. In fact, it is recommended in German Offenlegungsschrift No. 2,245,634 that such adducts or salt like compounds be used as catalyst in the production of foams having carbodiimide groups.